The N-formyl-methionyl peptides, F-Met-Leu-Phe, F-Met-Met-Phe and F-Met-Phe, when tested at differing concentrations, proportionally increased both in vitro cell locomotion and the expression of surface receptors for C3b on human peripheral blood neutrophils and monocytes. In contrast, the unformylated peptides, Met-Leu-Phe and Me-Met-Phe, had no chemotactic or complement receptor-enhancing activity at comparable concentrations. Casein and supernatants from human lymphocytes (cultured either in the presence or absence of phytohaemagglutinin), also recognized as chemotactic agents for human neutrophils and monocytes, enhanced C3b receptors on these cells in a similar dose-dependent fashion. These data, taken together with our previous findings with the eosinophil, suggest that in addition to promoting cell locomotion a further biological function of leucoattractants may be their capacity to render complement receptors more freely available thereby increasing the magnitude of adhesion of phagocytic cells to opsonized particles.